1. Technical Field
This invention relates generally to the field of negative index of refraction materials and devices using these materials.
2. Description of Related Technology
Material containing an electric permittivity and magnetic permeability that are simultaneously negative for some frequency have a negative index of refraction, and have been called left handed material (LHM) or negative index material (NIM). V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of ε and μ”, Soviet Phys. Usp. 10, 509 (1968), describes some characteristics of such a material, including a negative index of refraction and the ability to re-focus light passing through a thin slab composed of this material.
In the mid to late 1990s, John Pendry described some thin wires and split ring resonators (SRR) that paved the way to the fabrication of a meta-material that exhibited the negative index properties at microwave frequencies. J. B. Pendry, et. al., Phys. Rev. Lett., 76 4773 (1996) discloses that by using an array of thin metal wires, the plasma frequency of a metal could be shifted predictably to microwave frequencies. J. B. Pendry, et. al., IEEE Trans. Microw. Theory Techniques, 47 2075 (1999), discloses that by using an array of non-magnetic coupled metallic split ring resonators (SSR), the permeability of a metamaterial could be made to have negative values. This was demonstrated in the microwave by Smith in D. Smith, et. al., Phys. Rev. Lett., 84 4184 (2000), and more recently in the 100 terahertz range by Linden, in S. Linden, et. al., Science, 306, 1351 (2004). This structure uses split ring resonators and strip lines made of copper over circuit board material and is functional only at a single narrow band frequency, but demonstrated that microwave radiation passing through the wedged shaped NIM was bent through a large negative angle obeying Snell's Law, n1 sin θ1=n2 sin θ2. In such negative index materials, since n2 is negative, sin θ2 is also negative, yielding a large change in angle. In these structures, the size and spacing of the individual components comprising the metamaterial are assumed much smaller than the wavelength of the resonant frequency of operation. They are also fixed frequency structures.
Negative index of refraction material properties are disclosed in C. G. Parazzoli, et. al., Phys. Rev. Lett., 90 107401 (2003) A. A. Houck, et. al., Phys. Rev. Lett., 90 137401 (2003).
Intrinsically photoconductive materials such as gallium arsenide and silicon have been used as high frequency substrates, with roll offs of greater than 50-100 GHz for Si and 1 THz for GaAs, as described in P. Abele, et. al., IEEE MTT-S Digest, 1681 (2002), D. W. Van der Weild, Appl. Phys. Lett. 65, 881 (1994), and U. Bhattacharya, et. al, IEEE Microwave and Guided Wave Letters 5, 50 (1995). Photoconductive bridging of strip line waveguides and resonators on Si have been observed at frequencies as high as 15 GHz with as little as 1 milliwatt per square mm CW illumination at 870 nm, as described in Y. Horri and M. Tsutsumi, Asia Pacific Microwave Conf., 561 (1997). More recently, an IR-defined, photoconductive microwave bowtie antenna in Si exhibited turn-on characteristics at 0.1 watts/cm2 at 970 nm with metallic (copper) like behavior at 1 watt/cm2 CW illumination between 1-18 GHz, as described in D. Liu, et. al., IEEE Photon. Technol. Lett. Vol. 10, page 716 (1998).
Some negative index of refraction composite materials are described in U.S. Patent Publication No. 20070242360 to Rachford and U.S. Pat. No. 7,253,696 to Krowne, the entire disclosures of which are incorporated by reference herein.
Tonucci discloses photoconductive metamaterials with positive and negative tunable index of refraction and frequency in U.S. patent application Ser. No. 11/559,535 and U.S. patent application Ser. No. 11/559,508, both of which are incorporated by reference in their entireties.
U.S. Pat. No. 6,788,273 to Schultz et al. discloses a compensated radome for eliminating bore sight errors, in which the radome has a layer of negative index of refraction material and a layer of positive index of refraction material.